Earth moving equipment, such as a motor grader for example, often must operate in environments with poor footing conditions. Some earth moving machines are equipped with an all wheel drive system so that the front wheels of the machine, normally used for steering, are driven to help propel the machine in these poor footing conditions. Normally, only the rear wheels are driven, but when poor footing conditions are encountered causing excessive wheel slip, the front wheels of an all wheel drive machine can be driven to increase traction of the machine thereby reducing slip and maintaining directional control. When all wheels are in poor footing conditions, such as when working on side slopes, or maximum traction is desired, the machine can be operated in an all wheel drive mode. Operating a motor grader in an all wheel drive mode for maximum traction may be desirable when the motor grader is performing a cutting operation, such as cutting a new road or grading a side slope. The all wheel drive mode is also useful when a motor grader is used for removing snow from a roadway.
On a motor grader, a hydraulic pump is used to supply pressurized fluid to pump motors which drive the front wheels in the forward and reverse directions. A torque control lever is used in the all wheel drive mode to control the pressure and therefore the rim pull delivered to the front wheels. On initial engagement of the all wheel drive mode when the machine is moving, the torque control lever can cause the wheels to skid because the pump may not stroke rapidly enough to supply the flow of pressurized fluid required for the current wheel speed. The pump will supply fluid based upon the aggressiveness of the torque control lever which may not always match the current wheel speed. Accordingly, it will be appreciated, that it will be highly desirable to match the front wheel fluid flow, as the wheels are turning, to the flow the pump is generating before the two are connected together without regard to the aggressiveness of the torque control lever.